The just released Telerik RadControls Q3 Futures release (aimed as a public beta to the coming Q3 release) features a completely new rendering approach for the buttons that aimed to address problems like the ones you experience (e.g. with absolute positioning, or nested in relatively positioned parents, etc).
My suggestion would be to download the Q3 Futures and replace the Telerik.Web.UI dll - the particular issue that you report should be fixed.
the Telerik team
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